Heat-Index and Humidex

When heat and humidity combine…

Each of us, according to its thermoregulatory system, metabolism, body build, etc. “feel” the high temperatures in a different way than its neighbor. We are not all equal – in this area as in many others – and one could almost say that there are as many “feelings” of the heat as of human beings… There is nevertheless a factor of aggravation of the disagreeable sensation linked to the high temperatures that all the humans share: humidity! Indeed, at the same temperature, the higher the humidity is, the less our body will withstand this temperature, and the more it will send messages to inform us of its discomfort – and the real stress that the organism is undergoing.

It has now been about forty years that meteorologists have published indexes to measure – and predict – the effect of these high temperatures on our organisms. Often interpreted as feel-likes temperatures*, the two main standard indices, the Heat-Index and the Humidex are there to help us to anticipate the combined action of high temperatures and humidity.

Heat-index

This index, originally conceived by a weather presenter of an American television channel of the late 1970s, and then taken up and developed by the National Weather Service, represents the effect of humidity on an organism under severe heat conditions.

As we all know, the effect of sweating, by evaporation effect, is to cause surface cooling of the skin. Obviously, when humidity increases, the evaporation of sweating is less successful, the resulting cooling becoming weaker. The Heat-Index is used to represent this phenomenon: the higher the humidity is (at high constant temperature), the greater is the temperature-related discomfort. It is calculated relatively simply from relative humidity and atmospheric temperature.

with \({c}_{1}=-42,379\quad{c}_{2}=2,04901523\quad{c}_{3}=10,14333127\\{c}_{4}=-0,22475541\quad{c}_{5}=-6,83783.{10}^{-3}\quad{c}_{6}=-5,481717.{10}^{-2}\\{c}_{7}=1,22874.{10}^{-3}\quad{c}_{8}=8,5282.{10}^{-4}\quad{c}_{9}=-1,99.{10}^{-6}\)

and \({I}_{h}\) the heat-index, \({T}\) the atmospheric temperature in °F and \({H}\) the relative humidity (from 0 to 100).

This Heat-index is mainly used in the United States and is based on values expressed in imperial units, but nothing prevents you from converting it to the metric system after computation to have indices values close to the temperatures expressed in °C.

Humidex – or humidity index

The Humidex is an index used to indicate the effect of humidity on the body subjected to high temperatures. Developed by two engineers from the Meteorological Service of Canada in the same years as the Heat-Index, this index is now distinguished by its method of calculation which is based on the use of the dew point, rather than relative humidity.

with \({T}_{d}\) the dew point in °C et \({T}\) the atmospheric temperature in °C.

Humidex is mainly in use in Canada.

Warning and danger levels

You will find on the web all kinds of tables showing the warning and danger thresholds defined by these indices. I reproduce one of it, issued by the National Oceanic and Atmospheric Administration, regarding the Heat-index. You are free to interpret it as you wish. For my part I remain rather skeptical about this kind of table forgetting important elements such as the level of hydration and sun exposure…

(*) : Heat-index and Humidex are not feel-likes temperatures. These are indexes – values without units – which are close to the values of the usual temperatures, which is the reason for this confusion.